In recent years, light emitting diodes (LEDs) capable of emitting light with high luminance and high illuminating efficiency have been developed. In comparison with a common incandescent light, a LED has lower power consumption, long service life, and quick response speed. With the maturity of the LED technology, LEDs will replace all conventional lighting facilities. Until now, LEDs are widely used in many aspects of daily lives, such as automobile lighting devices, handheld lighting devices, backlight sources for LCD panels, traffic lights, indicator board displays, and the like.
When an electronic device (e.g. a LCD panel) having multiple LED strings is operated, the currents passing through all LED strings shall be identical for a purpose of obtaining uniform brightness. Due to different inherent characteristics of these LED strings, the currents passing therethrough are not identical and the brightness is usually not uniform. Therefore, the use life of individual LED string is shortened or even the whole electronic device has a breakdown.
For obtaining uniform brightness of multiple LED strings, several current sharing techniques have been disclosed. For example, referring to FIG. 1, a schematic circuit block diagram of a conventional LED current-supplying circuit is illustrated. As shown in FIG. 1, the LED current-supplying circuit 1 principally comprises a main power rectifying circuit 11, a primary DC-to-DC converting circuit 12, a first secondary DC-to-DC converting circuit 13a, a second secondary DC-to-DC converting circuit 13b, a first current-controlling circuit 14a and a second current-controlling circuit 14b. The main power rectifying circuit 11 is electrically connected to the primary DC-to-DC converting circuit 12. An AC input voltage Vin is rectified by the main power rectifying circuit 11 to generate a rectified input voltage Va. The input terminal of the primary DC-to-DC converting circuit 12 is electrically connected to the output terminal of the main power rectifying circuit 11. The output terminal of the primary DC-to-DC converting circuit 12 is electrically connected to the input terminals of the first secondary DC-to-DC converting circuit 13a and the second secondary DC-to-DC converting circuit 13b through a DC bus 16. By the primary DC-to-DC converting circuit 12, the rectified input voltage Va is converted into a bus voltage VDC having a specified DC voltage value. The first secondary DC-to-DC converting circuit 13a and the second secondary DC-to-DC converting circuit 13b are connected to the DC bus 16 for converting the bus voltage VDC into a first DC voltage V1 and a second DC voltage V2, which are required for powering at least one LED string. The first current-controlling circuit 14a is electrically connected to the output terminal of the first secondary DC-to-DC converting circuit 13a and the input terminal of a first LED string 15a. The second current-controlling circuit 14b is electrically connected to the output terminal of the second secondary DC-to-DC converting circuit 13b and the input terminal of a second LED string 15b. Under control of the first current-controlling circuit 14a and the second current-controlling circuit 14b, a first current I1 flowing through the first LED string 15a and a second current I2 flowing through the second LED string 15b have the same magnitude. As a consequence, the first LED string 15a and the second LED string 15b have the same brightness.
For providing the same current magnitude to each LED string, the secondary DC-to-DC converting circuits are indispensable to the conventional LED current-controlling circuits. Therefore, the overall circuitry cost is high but the current density is low. In the conventional LED current-supplying circuit, a specified bus voltage VDC is generated at the DC bus by the primary DC-to-DC converting circuit 12, and the bus voltage VDC is converted into regulated DC current required for the LED strings by the secondary DC-to-DC converting circuits. Under this circumstance, the conventional LED current-supplying circuit needs two-stage DC-to-DC converting circuits. Since there is a constant voltage drop between the input terminal and the output terminal of the secondary DC-to-DC converting circuit, the voltage drop causes power loss. Generally, the power loss is increased as the output voltage of the LED string is increased. In other words, the conventional LED current-supplying circuit has low efficiency, high circuitry cost and low current density.
There is a need of providing improved LED current-supplying circuit and LED current-controlling circuit to obviate the drawbacks encountered from the prior art.